It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk deactivates the marker by using a deactivation device provided to deactivate the marker.
One well-known type of marker (disclosed in U.S. Pat. No. 4,510,489) is referred as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized, it applies a bias magnetic field to the active element which causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency. The interrogation signal is generated by detecting apparatus, which also detects the resonance of the marker which is induced by the interrogation signal.
Magnetomechanical markers may be deactivated by exposing the bias element to an alternating magnetic field of sufficient magnitude to degauss the bias element. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus. One known type of deactivation device includes one or more coils energized by an alternating current signal to generate the deactivation magnetic field. An example of this kind of deactivator is disclosed in U.S. Pat. No. 5,341,125.
Other known types of deactivating apparatus include one or more arrays of permanent magnets arranged so that the polarities of the magnets are in alternating orientation along the array. The array of magnets provides a magnetic field that alternates in space along the array. When this magnetic field is applied to the bias element, the magnetic domain structure in the bias element is changed so that the bias element no longer provides the bias field required to place the active element in an activated condition. Deactivator devices which include arrays of permanent magnets are disclosed, for example, in U.S. Pat. No. 5,594,420, which has a common inventor and common assignee with the present application.
Both kinds of deactivators are often provided with a housing in the form of a low-profile pad structure having a flat top surface at which markers are presented for deactivation. The so-called "deactivator pads" are a common sight in retail stores.
One factor that may be considered in the design of marker deactivation devices is the possibility that items present in retail stores could be adversely affected by exposure to the magnetic field generated by the marker deactivation devices. Such items include videotapes, audiotapes, computer software diskettes, and particularly such items which have program material pre-recorded thereon. Moreover, credit cards and other bank cards typically have magnetic strips which may be erased or disrupted by the magnetic fields formed by marker deactivation devices.
According to one known approach to this problem, deactivation devices of the kind which include arrays of permanent magnets with alternating polarities are constructed using magnets that are quite small, so that the magnetic field falls off rather drastically with increasing distance from the magnet array. Accordingly, a marker carried on the outside of a tape cassette can be brought close to the magnet array for deactivation, while the magnetic tape inside the cassette is not exposed to a large field. A disadvantage of this approach is the need to bring the marker quite close to the magnet array if successful deactivation is to be assured.
The latter point brings up another design consideration for deactivation devices, namely that the device operate to reliably deactivate markers presented at some distance from the device, and without regard to the orientation of the marker relative to the deactivation device. The susceptibility of the marker bias element to being degaussed by an alternating magnetic field varies considerably depending on the orientation of the bias element relative to the alternating magnetic field. That is, an alternating field which is of sufficient strength to substantially demagnetize the bias element when applied along the length of the bias element may have little or no effect on the state of magnetization of the bias element if applied transversely to the length of the bias element. The problems of deactivating magnetomechanical markers at a distance from the deactivation device, and regardless of the orientation of the marker relative to the deactivation device, are addressed in U.S. patent application Ser. No. 08/794,012, filed Feb. 3, 1997, entitled "Multi-Phase Mode Multiple Coil Distance Deactivator for Magnetomechanical EAS Markers", which has common inventors and a common assignee with the present application. In typical prior art coil-based deactivation devices, an alternating magnetic field having a peak level of 550-600 Oe or more is provided at a top surface of the deactivation device to provide reliable deactivation at the top surface of the device and at some distance above the top surface.
In general, a trade-off must be made between the goal of deactivating magnetomechanical markers at a distance and without regard to marker orientation; on one hand, and avoiding undesirable effects on products which incorporate magnetic media, and credit cards, etc., on the other hand. The goal of deactivating markers at a distance and without regard to marker orientation is becoming particularly important because of the increasingly popular practice of "source tagging", i.e., securing markers to goods during manufacture or during packaging of the goods at a manufacturing plant or distribution facility. In some cases, the markers may be secured to the articles of merchandise in positions on or within the merchandise which make it difficult or impossible to bring the marker into contact with the surface of the deactivation device.
An invention disclosed in U.S. patent application Ser. No. 08/697,629, filed Aug. 28, 1996 (which has the same inventors and the same assignee as the present application), provides prospects for improving the trade off between reliable deactivation of markers and preventing harm to magnetically sensitive articles of merchandise. According to the '629 application, the bias element of a magnetomechanical marker is formed of a material having a lower coercivity than materials conventionally used in bias elements for magnetomechanical markers, so that reliable deactivation may be accomplished with lower field levels than have previously been employed. The disclosure of the '629 application is incorporated herein by reference.